Experimental models of colitis have shown that the microbiota plays a crucial role in disease, in that the mice develop colitis with a conventional microbiota but fail to develop intestinal inflammation when housed under germfree conditions. However, the signaling pathways by which the microbiota promotes colitis and the commensal bacteria that elicit inflammation in the intestine remain poorly understood. A major innate signaling pathway that can be activated by bacteria is the inflammasome, a multi-protein platform that activates caspase-1 leading to the proteolytic processing of pro-IL-1? and the release of IL-1?. The cytokine IL-1? is important for the regulation of innate and adaptive immune responses in colitis models including those induced by pathogenic organisms. Furthermore, IL-1? production by intestinal cells is associated with lesional disease activity in IBD patients, suggesting an important role for this cytokine in disease pathogenesis. However, the signaling pathways by which the microbiota promotes IL-1? production and the commensal bacteria that elicit IL-1? release in the intestine remain poorly understood. We find that the indigenous microbiota triggers activation of the NLRP3 inflammasome. Remarkably, selective members of the Enterobacteriaceae family and in particular Proteus mirabilis induce rapid and robust activation of the NLRP3 inflammasome and IL-1? release in vitro and in vivo. Importantly, intestinal colonization with P. mirabillis enhanced colitis via NLRP3. Based on these preliminary studies, we propose three specific Aims to test several hypotheses to understand the role of the microbiota in the induction of IL-1? via the NLRP3 inflammasome and the role microbiota- induced IL-1? in intestinal inflammation.